The present invention relates to a valve drive mechanism for an internal combustion engine for a portable, manually guided implement, especially a power chain saw.
Small four-stroke engines are known for portable, manually guided implements such as power chain saws, brush cutters, blowers, cut-off machines, or the like. Due to their type of construction with a valve drive mechanism, such engines have a greater overall size in the longitudinal axis of the cylinder than do port-controlled two-stroke engines. Portable, manually guided implements such as power chain saws have an implement housing with a rear handle, as viewed in the longitudinal direction of the housing, as well as a front, upper handle that is generally embodied as a tubular grip. The tubular handle extends form the region of the rear handle from one longitudinal side of the housing of the implement, over the upper side thereof, at an angle toward the front to a forward point of attachment, which is provided on the other longitudinal side of the housing. In order to be able to grip the handle with one hand, it must be appropriately spaced above the implement housing, i.e. above the internal combustion engine contained therein. In so doing, the tubular handle is disposed close to the cylinder head, so that due to the necessary free space relative to the handle a large cylinder requires a correspondingly greater path or orientation for the tubular handle. However, this results in a greater spacing of the handle relative to the center of gravity of the portable implement, which can adversely affect the ability to handle the implement. During use of a valve-controlled internal combustion engine, due to the valve drive that is disposed in the cylinder head the increased spacing of the tubular handle from the center of gravity must be accepted.
It is therefore an object of the present invention to provide a valve drive mechanism for a valve-controlled internal combustion engine such that when used in a portable implement a tubular handle that traverses the housing can be guided closer to the center of gravity of the implement.